Photovoltaic cells are used in a great variety of applications to convert electromagnetic radiation, e.g. the solar radiation, to electrical energy.
By a photovoltaic cell is meant here generally a semiconductor-based component converting incident electromagnetic radiation to electrical energy through a photovoltaic effect. In the case of solar radiation as the primary energy source, a photovoltaic cell is usually called a solar cell. In general, a photovoltaic cell comprises a p-type semiconductor region, an n-type semiconductor region, and an active region. The n-type and p-type semiconductor regions form a pn-junction. Photons of incident light having energy equal to or greater than the band gap energy of the active region material are absorbed, thereby generating free electrons and holes in the active material. These free charge carriers are then collected by means of electrodes connected to the different sides of the pn-junction.
One promising technological trend within the photovoltaic cell development is the field of thin film cells. In thin film photovoltaic cells the semiconductor layers of the device are realized as thin layers with a thickness in a range from a few nanometers to some tens of micrometers. Thin film cells can provide advantages in the energy conversion efficiency and in the manufacturing costs. Decreased layer thicknesses also mean lower weight of the cells and the solar panels formed of them. A thin overall structure enables also flexible device configurations. The active layer material of a thin film cell can be e.g. p-doped amorphous silicon or gallium arsenide.
In a photovoltaic cell, before any energy conversion can take place, light has to enter the cell and penetrate to the active region. Thus, losses due to reflection, scattering, and absorption before the active region should be minimized. Moreover, having reached the active region, the light energy should be absorbed there as effectively as possible without passing through it or reflecting back out of the cell. This is an important issue in all photovoltaic cell configurations. In thin film cells where the active region typically is a layer having a thickness in a range of only tens of nanometers to some micrometers, effective capturing of light energy into the active region becomes a key factor for the overall efficiency of the cell. On the other hand, light capturing solutions used with conventional thick film cells, e.g. different surface textures for coupling the incident light into the cell, are usually not suitable for thin film configurations.
Thus, intensive effort is focused in the field of thin film photovoltaic cells on research and development of more and more efficient solutions to improved light capturing.
Recently, one of the most active research areas has been the different forms of plasmonic light concentrators (LC) based on plasmonic nanoparticles arranged in an array on the surface of a photovoltaic cell on the side of the incident light. The operation of plasmonic nanoparticles (PNP) is based on resonant excitation of surface plasmons by the incident light. With suitable configuration of the nanoparticles and proper structural connection between the nanoparticles and the active region of the cell, the plasmon resonance, i.e. resonant oscillation of the electrons in the nanoparticles, results in efficient coupling of incident light into the active region. In this coupling, the light energy in the incident plane wave is concentrated into a plurality of so called hot spots located at least partially within the active region of the cell. At the same time, reflection backwards from the active region as well as transmission through it is minimized.
Many of the recently reported research activities in the field of plasmonic light concentrators are focused on metallic nanoparticles located on top of a thin film solar cell and covered. The metallic nanoparticles can be randomly distributed separate particles of silver or gold. These kinds of nanoparticles for light coupling are disclosed e.g. in US 2009/0250110 A1. Alternatively, a plasmonic light concentrator can be implemented as an array of regular two-dimensional nanoparticles like nanostrips as disclosed e.g. in EP 2109147 A1. The theory and principles of operation of both types of those nanoparticles and variations thereof are widely discussed in the scientific literature.
Common for both randomly organized nanoparticles and regularly arranged two-dimensional nanoparticles is that the wavelength/frequency band of the plasmonic enhancement is relatively narrow. With randomly distributed nanoislands, typically no more than about 10% of the wavelength/frequency band where photovoltaic conversion can take place in the active region is covered. With an array of nanostrips, the double-frequency operation due to two quadrupole plasmon resonances can typically provide a maximal coverage of about 20% of the available photovoltaic wavelength/frequency band. Forming a light concentrator as a double-periodic grid of nanostrips can increase the efficiency via the multi-frequency resonance thereby achieved up to half of the useful photovoltaic band.
So called nanoantennas (NA) arranged as an array on a photovoltaic cell provide an alternative way to implement plasmonic light concentrator arrangements. In a nanoantenna, the hot spot is created not due to excitation of a collective mode in the regular grid (called surface plasmon polariton) as is the case in a regularly arranged array of two-dimensional nanoparticles, but due to excitation of the eigenmodes of the antenna unit (called localized surface plasmons). Naturally, the plasmon resonance is influenced to some extent by the electromagnetic interaction between the neighboring antennas of the array, but the major factor is anyway the configuration of a single nanoantenna. One known nanoantenna configuration is the bow-tie nanoantenna consisting of two oppositely placed substantially triangular nanopatches. The local field at the plasmon resonance is concentrated in the gap between the apexes of the triangles. The center of this hot spot is located inside the substrate on which the nanoantennas are formed. The displacement of the hot spot into the substrate results from its higher permittivity compared to that of the free space. Another known nanoantenna type is the dimer type configuration comprising two adjacent circular nanopatches. The design and operation principles of both bow-tie and dimer type nanoantennas are widely discussed in the scientific literature.
An important feature of nanoantennas is that any strict antenna array regularity is not required. Moreover, single antenna units are not very sensitive to the manufacturing tolerances thereof. The more tolerant geometrical dimensions of a nanoantenna array allow use of manufacturing equipment with lower cost than e.g. the electron or ion beam lithography usually required for manufacturing regularly distributed arrays of two-dimensional plasmonic nanoparticles. Thus, from industrial-scale manufacturing point of view, nanoantennas provide a very promising approach to implement plasmon enhanced photovoltaic cells. On the other hand, also the known nanoantenna configurations suffer from a narrow wavelength/frequency band of the plasmon resonance. For bow-tie antennas, the dipole-type resonance band typically covers only about 5% of the available band of the photovoltaic effect.
As an approach entirely different from the plasmonic light concentrators, it is known to enhance the light capturing efficiency of a photovoltaic cell by placing the active region within a resonant cavity, usually called a standing wave Fabry-Perot resonant cavity. In general, such a resonant cavity is formed by two refracting, substantially lossless layers located at opposite sides of the active region to serve as reflecting end elements of the cavity. The cavity is designed to form a standing wave confined in the thus formed cavity. Resonance enhances the optical field within the active region and thus increases the light capturing efficiency. The energy which is absorbed in the active region is reimbursed by the incident light flux. A standing wave Fabry-Perot cavity can be manufactured with lower costs than e.g. the more complicated regular plasmonic light concentrators. However, the light capturing enhancement covers again a very narrow portion of the available wavelength/frequency range of the photovoltaic effect, typically 5-7%. More broadband standing wave Fabry-Perot type cavities are also known. However, they comprise complex multilayer structures located on both sides of the active photovoltaic region, the multilayer structures necessitating nanometer-scale precision of manufacture. The costs of such cavities are comparable with the costs of the regular plasmonic light concentrators, thus making them unsuitable for use in typically very cost-critical thin-film solar cells.
As is clearly seen in the prior art description above, there is a strong demand in the field of thin film photovoltaic cell structures for more efficient light capturing solutions which, preferably, could be manufactured with reasonable manufacturing costs.